The present invention relates to particles, pellets or granules of fine-particle or nanoparticle iron oxides and/or iron oxyhydroxides having a large specific surface area (50 to 500 m2/g according to BET), and processes for their production. These pellets have high mechanical resistance and can be used as a contact, adsorbent, or catalyst for the catalysis of chemical reactions, for the treatment of fluid media like liquids and/or for gas, specifically the removal of impurities.
Contact and adsorbent granules, including those based on iron oxides and/or iron oxyhydroxides, have already been described. They are predominantly used in continuous processes. They are conventionally found in tower or column-type units through which the medium to be treated flows, and the chemical or physical reaction or adsorption processes take place at the outer and inner surface of the granules. Powdered materials cannot be used for this purpose because they compact in the direction of flow of the medium, thereby increasing the flow resistance until the unit becomes blocked. If a unit is cleaned by back-flushing (see below), large amounts of the powder are discharged and lost or cause an unacceptable contamination of the waste water.
The flowing media also exert forces on the granules, however, which can lead to abrasion and/or movement through to violent agitation of the granules. Consequently the granules collide, leading to undesirable abrasion. This results in loss of contact or adsorbent material and contamination of the medium to be treated.
Adsorbents/catalysts containing iron oxides and hydroxides can advantageously be used e.g. in the area of water purification or gas purification. In water purification this agent is used in horizontal- or vertical-flow filters or adsorber columns or added to the water to be treated in order to remove dissolved, suspended or emulsified organic or inorganic phosphorus, arsenic, antimony, sulfur, selenium, tellurium, beryllium, cyano and heavy metal compounds from, for example, drinking water, process water, industrial and municipal waste water, mineral, holy and medicinal water as well as garden pond and agricultural water. It can also be used in so-called reactive walls to separate the cited contaminants from ground water and seepage water aquifers from contaminated sites (waste disposal sites).
In gas purification the agent is used in adsorbers for binding undesirable components such as hydrogen sulfide, mercaptans and hydrogen cyanide, as well as other phosphorus, arsenic, antimony, sulfur, selenium, tellurium, cyano and heavy metal compounds in waste gases. Gases such as HF, HCl, H2S, SOx, NOx can also be adsorbed.
The removal of phosphorus, arsenic, antimony, selenium, tellurium, cyano and heavy metal compounds from waste oils and other contaminated organic solvents is also possible.
Contact and adsorbent granules based on iron oxides and/or iron oxyhydroxides are also used for the catalysis of chemical reactions in the gas phase or in the liquid phase.
Various methods of removing trace constituents and contaminants from aqueous systems with the aid of adsorbents are also known.
For example, DE-A 3 120 891 describes a process in which a filtration is performed using activated alumina with a grain size of 1 to 3 mm for the separation principally of phosphates from surface water.
DE-A 3 800 873 describes an adsorbent based on porous materials such as e.g. hydrophobed chalk with a fine to medium grain size to remove contaminants from water.
DE-A 3 703 169 discloses a process for the production of a granulated filter medium to treat natural water. The adsorbent is produced by granulating an aqueous suspension of kaolin with addition of powdered dolomite in a fluidised bed. The granules are then baked at 900 to 950° C.
A process for the production and use of highly reactive reagents for waste gas and waste water purification is known from DE-A 40 34 417. Mixtures consisting of Ca(OH)2 with additions of clays, stone dust, entrained dust and fly ashes, made porous and having a surface area of approx. 200 m2/g, are described here.
The cited processes have the disadvantage that the component responsible in each case for the selective adsorption of constituents of the media to be cleaned, in other words the actual adsorbent, must be supplemented with large quantities of additives to enable it to be shaped into granules. This significantly reduces the binding capacity for the water contaminants to be removed. Moreover, subsequent reprocessing or reuse of the material is problematic since the binder substances first have to be separated out.
DE-A 4 214 487 describes a process and a reactor for the removal of impurities from water. The medium flows horizontally through a funnel-shaped reactor, in which finely divided iron hydroxide in flocculent form is used as a sorption agent for water impurities. The disadvantage of this process lies in the use of the iron hydroxide in flocculent form, which means that because there is little difference in density between water and iron hydroxide, a reactor of this type can be operated at only very low flow rates and there is a risk of the sorption agent, which is possibly already loaded with contaminants, being discharged from the reactor along with the water.
JP-A 55 132 633 describes granulated red mud, a by-product of aluminium production, as an adsorbent for arsenic. This consists of Fe2O3, Al2O3 and SiO2. No mention is made of the stability of the granules or of the granulation process. A further disadvantage of this adsorbent is the lack of consistency in the composition of the product, its unreliable availability and the possible contamination of the drinking water with aluminium. Since aluminium is suspected of encouraging the development of Alzheimer's Disease, contamination with this substance in particular is to be avoided.
DE-A 19 826 186 describes a process for the production of an adsorbent containing iron hydroxide. An aqueous polymer dispersion is incorporated into iron hydroxide in water-dispersible form. This mixture is then either dried until it reaches a solid state and the solid material then comminuted mechanically to the desired shape and/or size or the mixture is shaped, optionally after a preliminary drying stage, and a final drying stage then performed, during which a solid state is achieved. In this way a material is obtained in which the iron hydroxide is firmly embedded in the polymer and which is said to display a high binding capacity for the contaminants conventionally contained in waste waters or waste gases.
The disadvantage of this process lies in the use of organic binders, which further contaminate the water to be treated due to leaching and/or abrasion of organic substances. Furthermore, the stability of the adsorbent composite is not guaranteed in extended use. Bacteria and other microorganisms can also serve as a nutrient medium for an organic binder, presenting a risk that microorganisms may populate the contact and thereby contaminate the medium.
The presence of foreign auxiliary substances, which are required for the manufacture of the adsorbents, during reprocessing, recycling or reuse of used adsorbents is disadvantageous in principle because the reuse of pure substances is less problematic than is the case with mixed substances. For example, polymeric binders are disadvantageous when iron oxide-based adsorption materials are reused as pigments for concrete coloration because these binders inhibit dispersion of the pigment in liquid concrete.
DE-A 4 320 003 describes a process for the removal of dissolved arsenic from ground water with the aid of colloidal or granulated iron hydroxide. Where fine, suspended iron(III) hydroxide products are used, it is recommended here that the iron hydroxide suspension be placed in fixed-bed filters filled with granular material or other supports having a high external or internal porosity. This process likewise has the disadvantage that, relative to the adsorbent “substrate+iron hydroxide”, only low specific loading capacities are achievable. Furthermore, there is only a weak bond between substrate and iron hydroxide, which means that there is a risk of iron hydroxide or iron arsenate being discharged during subsequent treatment with arsenic-containing water. This publication also cites the use of granulated iron hydroxide as an adsorption material for a fixed-bed reactor. The granulated iron hydroxide is produced by freeze conditioning (freeze drying) of iron hydroxide obtained by neutralisation of acid iron(III) salt solutions at temperatures of below minus 5° C. This production process is extremely energy-intensive and leads to heavily salt-contaminated waste waters. Moreover, as a result of this production process only very small granules with low mechanical resistance are obtained. When used in a fixed-bed reactor, this means that the size spectrum is significantly reduced by mechanical abrasion of the particles during operation, which in turn results in finely dispersed particles of contaminated or uncontaminated adsorption agent being discharged from the reactor. A further disadvantage of these granules lies in the fact that the adsorption capacity in respect of arsenic compounds is reduced considerably if the granules lose water, by being stored dry for extended periods for example.
Adsorbent/binder systems obtained by removing a sufficiently large amount of water from a mixture of (a) a crosslinkable binder consisting of colloidal metal or non-metal oxides, (b) oxidic adsorbents such as metal oxides and (c) an acid such that components (a) and (b) crosslink to form an adsorbent/binder system, are known from U.S. Pat. No. 5,948,726. According to the disclosure, colloidal alumina or aluminium oxide is used as binder.
The disadvantage of these compositions lies in the need to use acid in their production (column 9, line 4) and in the fact that they are not pure but heterogeneous substances, which is undesirable both for the production, regeneration, removal and permanent disposal of such adsorbents, e.g. on a waste disposal site. The scope of disclosure of this publication is also intended to include adsorbents that are suitable for the adsorption of arsenic; specific examples are not provided, however. Aluminium oxide is known to be significantly inferior to iron oxides in regard to force of adsorption for arsenic.
Continuous adsorbers, which are commonly grouped together in parallel for operation, are preferably used for water treatment. To free drinking water from organic impurities, for example, such adsorbers are filled with activated carbon. At peak consumption times, the available adsorbers are then operated in parallel to prevent the flow rate from rising above the upper limit permitted by the particular arrangement. At times of lower water consumption, individual adsorbers are taken out of operation and can be serviced, for example, whereby the adsorption material is subjected to special loads, as described in greater detail below.
The use of granules, which can be produced by compacting e.g. powdered iron oxide using high linear forces, has also already been considered. Such granules have already been described as a means of homogeneously colouring liquid concrete. The use of high linear forces for compacting is extremely expensive and energy-intensive, and the stability of the compacted materials is inadequate for extended use in adsorbers.
The use of such materials in adsorbers, for example, particularly continuous models, for water purification is therefore of only limited interest. During maintenance or cleaning of adsorber plants by back-flushing in particular (see below), such granules lose large amounts of substance due to the associated agitation. The abraded material renders the waste water from back-flushing extremely turbid. This is unacceptable for a number of reasons: firstly, adsorption material, which is heavily laden with impurities and therefore toxic after extended use, is lost. Secondly, the stream of waste water is laden with abraded material, which can sediment, damaging piping systems and ultimately subjecting the waste treatment plant to undesirable physical and toxicological stresses, to name but a few reasons. Preferably the abrasion should be below 20% by weight, more preferably below 15% by weight, 10% by weight or most preferably below 5% by weight according to the method described in the examples of the present invention.
An object of the present invention was therefore to provide a contact or an adsorbent/catalyst based on iron-oxygen compounds in pellet form, exhibiting high mechanical resistance in conjunction with a good binding capacity for contaminants contained in liquids and gases without the need to use organic binders or inorganic foreign binders to achieve adequate mechanical resistance, and plants operated with such media. This object is achieved by the contacts or adsorbents/catalysts according to the invention, their preparation, their use and the units filled therewith.